Adsorbent of latent-heat storage type for canister and process for producing the same

ABSTRACT

The present invention provides a latent-heat storage type adsorbent composition for canisters that can effectively prevent changing in temperature due to the heat of absorption/desorption and has a high butane working capacity; a process for producing the adsorbent; and a canister employing the latent-heat storage type adsorbent composition for canisters. The present invention relates to a latent-heat storage type adsorbent composition for canisters which compromises an adsorbent adsorbing a fuel vapor and a heat-storage material comprising microencapsulated phase-changing material which absorbs or releases latent heat in response to temperature change, and a method for producing such an adsorbent.

TECHNICAL FIELD

The present invention relates to a canister, i.e., an apparatus forpreventing vehicle fuel from vaporizing, and an adsorbent compositionused therein.

BACKGROUND ART

As an anti-pollution measure, the fuel vapor that is generated in anautomobile's fuel storage chambers, such as the fuel tank, the floatchamber of the carburetor, etc., is usually guided to a carbon canisterduring stops and while traveling and adsorbed by activated carbon, whichserves as an adsorbent. While traveling, air is brought into thecanister to desorb the adsorbed fuel, and the desorbed fuel is fed intoan inlet pipe of the engine through a control valve.

Generally, the lower is the temperature of the activated carbon, thegreater is the fuel vapor adsorption capacity of the activated carbon.Conversely, the higher is the temperature of the activated carbon, thegreater is its desorption capacity. However, because the reaction inwhich fuel vapor is adsorbed by activated carbon is an exothermicreaction with the result that the temperature of the activated carbonrises as fuel vapor is adsorbed, the adsorption capacity of theactivated carbon is invariably decreased. In contrast, because thereaction in which the fuel vapor is desorbed from the activated carbonis an endothermic reaction with the result that the temperature of theactivated carbon is lowered as fuel vapor is desorbed, the desorptioncapacity of the activated carbon is also decreased.

To solve the above problem, a canister is proposed in which a granularmaterial having a specific heat greater than that of activated carbon ismixed with the activated carbon. In this canister, the increase in thetemperature of the activated carbon caused by the heat generated by thefuel vapor adsorption from the activated carbon is attenuated by thepresence of a material having a greater specific heat; conversely, theheat stored in the material having a greater specific heat is utilizedto supply the heat that is required for desorbing the fuel vapor fromthe activated carbon, to prevent the decrease in the temperature of theactivated carbon. Thus, the adsorption/desorption characteristics areimproved.

However, materials having a high specific heat composed of metals,ceramics, etc., whose specific heat is low relative to theabsorption/desorption heat, must be used in a large amount incombination with the activated carbon to obtain a satisfactory effect.Combined use of activated carbon with these materials having littleadsorption capacity will not greatly improve the adsorption capacity asa whole, even if the temperature aspect is improved.

DISCLOSURE OF THE INVENTION

To solve or alleviate the above problems, an object of the presentinvention is to provide a latent-heat storage type adsorbent compositionfor canisters that can effectively prevent changes in temperature due toabsorption/desorption heat and has a high butane working capacity; aprocess for producing such an adsorbent composition; and a canisteremploying the latent-heat storage type adsorbent composition.

The present inventors conducted an extensive research and found that theabove object can be achieved by an adsorbent composition for canistersthat includes an adsorbent capable of adsorbing fuel vapors and apowdery heat-storage material comprising a microencapsulatedphase-change material, wherein the phase-change material absorbs orreleases latent heat in response to temperature change. The presentinvention is accomplished based on these novel findings.

In more detail, the present invention provides the following latent-heatstorage type adsorbent composition for canisters, a method for producingthe same, and a canister for preventing the vaporization of fuel.

1. A latent-heat storage type adsorbent composition for canisterscomprising an adsorbent and a heat-storage material;

the adsorbent being capable of adsorbing fuel vapor,

the heat-storage material comprising a microencapsulated phase-changematerial, the phase-change material absorbing or releasing latent heatin response to temperature change.

2. A latent-heat storage type adsorbent composition for canistersaccording to Item 1, wherein the adsorbent is activated carbon,activated alumina or a mixture thereof.

3. A latent-heat storage type adsorbent composition for canistersaccording to Item 1 or 2, wherein the average particle diameter of theheat-storage material is about 1/1000 to about 1/10 of that of theadsorbent.

4. A latent-heat storage type adsorbent composition for canistersaccording to Item 1, 2 or 3, wherein the average particle diameter ofthe adsorbent is about 1 μm to about 10 mm.

5. A latent-heat storage type adsorbent composition for canistersaccording to any one of Items 1 to 4, wherein the average particlediameter of the heat-storage material is about 0.1 to about 500 μm.

6. A latent-heat storage type adsorbent composition for canistersaccording to any one of Items 1 to 5, wherein the heat-storage materialis adhered to and/or deposited on the surface of the adsorbent.

7. A latent-heat storage type adsorbent composition for canisters whichis in a form of a molded article comprising a latent-heat storage typeadsorbent composition for canisters according to any one of Items 1 to 6and a binder.

8. A latent-heat storage type adsorbent composition for canistersaccording to Item 7, wherein the molded article is in at least one shapeselected from the group consisting of pellet, disc and block.

9. A method for producing a latent-heat storage type adsorbentcomposition for canisters according to any one of Items 1 to 6 whereinthe heat-storage material is adhered to and/or deposited on the surfaceof the adsorbent.

10. A method for producing a latent-heat storage type adsorbentcomposition for canisters according to any one of Items 1 to 6 whereinthe heat-storage material is electrostatically adhered to and/ordeposited on the surface of the adsorbent.

11. A method for producing a latent-heat storage type adsorbentcomposition for canisters according to any one of Items 1 to 6 whereinthe heat-storage material and the adsorbent are uniformly mixed.

12. A method for producing a latent-heat storage type adsorbentcomposition for canisters according to any one of Items 1 to 6 wherein aslurry obtained by suspending the heat-storage material in a liquidmedium is mixed with the adsorbent, and the mixture is then dried.

13. A method for producing a latent-heat storage type adsorbentcomposition for canisters comprising:

suspending a heat-storage material containing a microencapsulatedphase-change material in a liquid medium to give a slurry, thephase-change material capable of absorbing or releasing latent heat inresponse to temperature change, and

spraying a liquid mixture containing the slurry and, if necessary, abinder, on the surface of the fuel vapor adsorbent.

14. A method for producing a latent-heat storage type adsorbentcomposition for canisters comprising:

molding a heat-storage material containing a microencapsulatedphase-change material capable of absorbing or releasing latent heat inresponse to temperature change to produce a molded article, and

uniformly mixing a fuel vapor adsorbent and the molded article.

15. A method for producing a latent-heat storage type adsorbentcomposition for canisters comprising:

uniformly mixing a fuel vapor adsorbent, a powdery heat storage materialcontaining a microencapsulated phase-change material capable ofabsorbing or releasing latent heat in response to temperature change ora slurry suspending the powdery heat storage material in the liquidmedium, a binder and water, and

molding the resultant mixture to form a desired shape.

16. A latent-heat storage type adsorbent composition for canistersobtained by the method according to any one of Items 13 to 15.

17. A canister for preventing fuel vaporization in which the latent-heatstorage type adsorbent composition of any one of Items 1 to 8 and 16 isplaced in a canister case.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention is described in detail.

Latent-heat Storage Type Adsorbent Composition for Canisters

The latent-heat storage type adsorbent composition for canisters of thepresent invention can be obtained by mixing an adsorbent capable ofadsorbing fuel vapor (hereinafter sometimes referred to as simply an“adsorbent”) and a powdery heat-storage material comprising amicroencapsulated material that absorbs or releases latent heat inresponse to temperature change and undergoes phase changes (Hereinafterreferred to as a “phase-change material”). More precisely, thelatent-heat storage type adsorbent composition for canisters of theinvention has a feature that a heat-storage material comprising aphase-change material is used for controlling the heat generated in theadsorbent for use in a canister.

The latent-heat storage type adsorbent composition for canisters isapplied to fuel vapors such as automobile gasoline vapor, etc.

The adsorbent for adsorbing fuel vapor used in the invention may be anadsorbent used in known canisters; examples include activated carbon,activated alumina, silica gel, zeolites, organometallic complexes,silica porous bodies, etc., and mixtures thereof. Activated carbon,activated alumina and mixtures thereof are preferable. Activated carbonis particularly preferable. Activated carbon can be those obtained fromvarious materials such as coal, coconut shell, wood, lignin, etc.; thesematerials can be converted into activated carbon by water vapor; carbondioxide; phosphoric acid, zinc chloride, alkali metals or likeindustrial chemicals.

The adsorbents used in the invention are preferably in the form ofgranules or powder having micro pores to increase the capacity foradsorbing fuel vapor. The average particle diameter of the adsorbent isusually in the range of about 1 μm to about 10 mm. The specific surfacearea thereof is usually about 500 to about 2500 m²/g and preferablyabout 800 m²/g to about 2300 m²/g. The diameter of the micro pore isusually about 10 Å to about 50 Å and preferably about 10 Å to about 35Å.

The heat-storage material used in the invention comprises microcapsulepowder in which a phase-change material is encapsulated.

There is no limitation to the phase-change material that is encapsulatedin the heat-storage material as long as it can adsorb and release latentheat with a change from one phase to another. The phase change can be achange between a solid phase and a liquid phase, etc. The temperature atwhich the phase-change material starts to undergo a phase change (e.g.,melting point, solidifying point, etc.) can be suitably adjusteddepending on the usage of the canister, and is generally about 0° C. toabout 50° C. Preferable examples of phase change compounds aretetradecane, pentadecane, hexadecane, heptadecane, octadecane,nonadecane, eicosane, docosane and like straight-chain aliphatichydrocarbons; natural waxes; petroleum waxes; LiNO₃.3H₂O, Na₂SO₄.10H₂O,Na₂HPO₄. 12H₂O and like hydrates of inorganic compounds; capric acid,lauric acid and like fatty acids; C₁₂₋₁₅ higher alcohols; methylpalmitate, methyl stearate and like esters, etc. To control the meltingpoint of the phase-change material, a combination of two or morecompounds selected from the above may be used. When two or morephase-change materials are used in combination, it is preferable thatthe difference in the temperature at which the selected phase-changematerials undergo phase change is in the range of about 0° C. to about15° C.

To prevent supercooling in the phase-change material, a compound havinga melting point higher than that of the phase-change material may beadded. Examples of such high melting point compounds are aliphatichydrocarbons, aromatic compounds, esters, carboxylic acids, alcohols,amides, etc. These high melting point compounds may be used singly or incombination of two or more. Mixtures such as castor oil, etc., are alsousable.

Examples of aromatic compounds include halogen-substituted benzenes,naphthalene, etc. Examples of halogen-substituted benzenes includedibromobenzene, dichlorobenzene and like dihalogenated benzenes.

Examples of esters include fatty acid esters of monoalcohols such asmethyl eicosanoate; fatty acid esters of glycerol such as glycerides oflinoleic acid, etc.

Examples of carboxylic acids include myristic acid, pentadecylic acid,palmitic acid, margaric acid, stearic acid, nonadecanoic acid,eicosanoic acid, henicosanoic acid, behenic acid and like aliphaticcarboxylic acids; benzoic acid and like aromatic carboxylic acids, etc.

Examples of alcohols include cetyl alcohol, heptadecanol, stearylalcohol, nonadecanol, eicosanol and like monoalcohols.

Examples of amides include eicosanamide, nonadecanamide, stearamide,oleamide and like fatty acid amides.

The concentration of additive high melting point compound is usuallyabout 0.5 wt % to about 30 wt % and preferably about 1 wt % to about 15wt %, relative to the phase-change material.

Known materials can be used as materials for the microencapsulatingphase-change material, such as polymer compounds such as resins.Examples of polymer compounds include formaldehyde/melamine resins,melamine resins, formaldehyde/urea resins, urea resins,urea/formaldehyde/polyacrylic acid copolymers, polystyrene, polyvinylacetate, polyacrylonitrile, polyethylene, polybutyl methacrylate,gelatin, etc.

The weight ratio of the microcapsule material to the phase-changematerial is not limited and usually in the range of about 30:70 to10:90. When a high melting point compound and a phase-change materialare used in combination, the weight ratio of the microcapsule materialto the total amount of high melting point compound and phase-changematerial can be set in the above range.

The phase-change material used in the invention can be microencapsulatedby a known method, such as coacervation methods, interfacialpolymerization methods, in-situ methods, methods using yeast, etc. Theeffects of the invention can be attained by any of the known methods.

For example, it is possible to prepare a dispersion (slurry) ofmicrocapsules having resin walls and the phase-change material (and, ifnecessary, a high melting point compound) enclosed therein, byemulsifying a phase-change material (and, if necessary, a high meltingpoint compound) in a liquid medium using an emulsifier or the like,adding an initial condensate (prepolymer) corresponding to the desiredresin, raising the temperature of the mixture to finish thepolymerization reaction.

As a liquid medium, water is particularly preferable, and water misciblesolvent, such as methanol, ethanol, propanol and like alcohols, acetone,etc., may also be used. It is also possible to use a mixture of theabove-mentioned solvents.

The microcapsules are generally spherical particles; and the conditionssuitable for controlling the diameter of the particles are variabledepending on the kind and concentration of the emulsifier used for theencapsulation, the temperature and duration of emulsification, themethod for emulsification, etc., and therefore the most preferableconditions can be suitably determined by experimentation. The averageparticle diameter of the microcapsule is usually about 1/1000 to about1/10 of that of the adsorbent, in consideration of the contact area overthe microcapsules with the adsorbent. Specifically, it is usually in therange of about 0.1 μm to about 500 μm and preferably about 0.5 μm toabout 500 μm.

The latent-heat storage type adsorbent composition for canisters of theinvention is a mixture in which an adsorbent and a heat-storage materialcomprising a microencapsulated phase-change material are distributeduniformly, and the heat-storage material is adhered to the surface ofthe adsorbent particles.

Method for Producing a Latent-heat Storage Type Adsorbent Compositionfor Canisters

The latent-heat storage type adsorbent composition for canisters of thepresent invention can be produced, for example, by the following manner.The objective powdery composition can be obtained by uniformly mixing anadsorbent with a dispersion (slurry) of microcapsules comprising aphase-change material, wherein the dispersion (slurry) is obtained bythe above-mentioned method, etc., and then drying the resulting mixture.

Alternatively, it is possible to obtain the objective powderycomposition by uniformly mixing an adsorbent with a microcapsule powder(heat-storage material) obtained by drying a dispersion (slurry) ofmicrocapsules.

The mixing method mentioned above can be selected from the knownmethods, such as placing the heat-storage material (or slurry) and theadsorbent in a predetermined case or bag and shaking the mixture;methods using a mixer, kneader and other stirrers; methods using arotary mixer, etc. The drying method can also be selected from amongknown methods.

The latent-heat storage type adsorbent composition for canisters of theinvention preferably has high heat transfer efficiency, because aheat-storage material having a diameter smaller than that of anadsorbent is adhered to the surface of the adsorbent particles, and theheat-storage material and the adsorbent can contact each other. Forexample, by controlling the average particle diameters of theheat-storage material and the adsorbent in the manner described above,the heat-storage material can electrostatically adhere to or deposit onthe surface of the adsorbent merely by uniformly mixing the heat-storagematerial with the adsorbent, and therefore the latent-heat storage typeabsorbent composition for canister has high packing density and highheat transfer efficiency. Furthermore, because separation of theheat-storage material and the adsorbent can be lessened, it is possibleto prevent changes in temperature during adsorption and desorptioncycles for a long time.

To prevent the adsorbent from being released from the canister into theengine, the powdery latent-heat storage type adsorbent composition forcanisters of the invention can be formed into molded articles. Moldingcan be conducted by a known method, such as mixing a powderyheat-storage material with adsorbent and compression molding themixture, etc.

If necessary, the latent-heat storage type adsorbent composition forcanisters can be molded with a binder. The molded articles can beobtained by, for example, uniformly mixing a heat-storage material,adsorbent, and binder in the liquid medium, adhering and/or depositingthe heat-storage material on the surface of the adsorbent, and thenmolding the mixture. Usable binders are not limited, and examplesthereof are generally used ones such as methylcellulose,carboxymethylcellulose and like celluloses; phenol resins; polyvinylalcohol; vinyl acetate, etc. Examples of shapes of the molded articlesinclude pellets, discs, blocks, etc.

Example of a production method other than that exemplified above is asfollows: The latent-heat storage type adsorbent composition of theinvention wherein the microcapsules coat the surface of the adsorbentcan be obtained by spraying a mixture of a dispersion containing amicroencapsulated phase-change material (a slurry suspending theheat-storage material in a liquid medium) and, if necessary, a binder,onto the surface of the adsorbent having the shape of a pellet,pulverized powder, etc., and drying the sprayed adsorbent. Examples ofbinders are methylcellulose, carboxymethylcellulose and like celluloses;phenol resins; polyvinyl alcohol; vinyl acetate and like known binders.Known methods can be employed for mixing, spraying and drying.

Alternatively, it is possible to obtain the latent-heat storage typeadsorbent composition of the invention by uniformly mixing adsorbenthaving the shape of pellets or a pulverized powder with microcapsules(heat-storage material) molded in a certain shape such as cylindricalpellets, spherical pellets, sheets, etc. A known method can be employedto form the microcapsules (heat-storage material), and, if necessary, abinder may be added when molding the heat-storage material. Theabove-mentioned known binders can be used as binder.

Alternatively, it is possible to obtain the latent-heat storage typeadsorbent composition of the invention by uniformly mixing adsorbentpowder, a powder or other form of heat-storage material or amicrocapsule dispersion liquid containing a phase-change material (aslurry suspending the heat-storage material in a liquid medium), abinder, and water; and molding the mixture. The above-mentioned knownbinders can be used and a known molding method can be employed.

The ratio of the heat-storage material to the adsorbent contained in thelatent-heat storage type adsorbent composition of the invention can besuitably selected by a person skilled in the art depending on theproperties thereof. The content of the heat-storage material is usuallyabout 10 to about 100 parts by weight based on 100 parts by weight ofthe adsorbent. When a binder is used, the content of the binder isgenerally about 1 to about 10 parts by weight based on 100 parts byweight of the adsorbent.

The adsorbent composition of the invention placed in a canister case canadsorb fuel vapor gas by introducing the fuel vapor gas from a fuel tankinto the case. The temperature of the gas and the case is preferably nothigher than the temperature at which the phase-change material undergoesa phase change (usually the melting point).

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention is explained in great detail withreference to Examples and Comparative Examples. Note that the presentinvention is not limited to or by these Examples.

EXAMPLE 1

To 5 g of melamine powder, were added 6.5 g of 37% aqueous formaldehydesolution and 10 g of water, the pH of the mixture was adjusted to pH 8,and the temperature was raised to about 70° C., giving an aqueoussolution of melamine/formaldehyde initial condensate. Separately, asolution of 70 g of n-octadecane as a phase-change compound was added to100 g of an aqueous solution of a styrene anhydride copolymer sodiumsalt adjusted to pH 4.5 while intensely stirring, and emulsification wasconducted until the particle diameter became about 10 μm. Encapsulationwas conducted by adding the total amount of the above aqueous solutionof melamine/formaldehyde initial condensate to the thus obtainedemulsified solution, stirring at 70° C. for two hours, and adjusting topH 9.

After completion of the reaction, the capsules were filtered out bysuction and dried, giving capsules having a particle diameter of about15 μm. These capsules were uniformly mixed with pulverized activatedcarbon having a particle diameter of 0.2 mm to 3 mm in such a mannerthat the proportion of the capsules relative to the activated carbon was15 wt %, and the resulting mixture was then dried, obtaining theobjective heat-storage type adsorbent composition wherein themicrocapsules were dispersed over the surface of the activated carbon.

The activated carbon used was 0.50 mm to 2.36 mm pulverized coalactivated carbon having a specific surface area of 1250 m²/g, a porecapacity of 0.71 ml/g, and an average pore diameter of 12 Å.

EXAMPLE 2

The butane working capacity of the heat-storage type adsorbentcomposition obtained in Example 1 was measured by the following method.The above heat-storage type adsorbent composition was placed in aone-liter metal canister, 99% n-butane was adsorbed by downflowing it tothe canister at one l/min at 25° C., and supply of the butane wasstopped when the concentration of the butane at the outlet reached 5000ppm. Air was then let into the canister at room temperature at 15 l/minfor 20 minutes by upflowing it to the canister to desorb the n-butane.Such absorption and desorption operations were repeated. The butaneworking capacity was determined based on the mean values of the amountsof the 4 ^(th), 5 ^(th) and 6 ^(th) absorption and desorption cycles.

As a result, the butane working capacity was found to be 46.7 g/l for aone-liter canister case.

COMPARATIVE EXAMPLE 1

The butane working capacity of activated carbon used in Example 1 onlywas measured in the same manner as in Example 2, and found to be 41.6g/l for a one-liter canister case.

As is clear from the above result, the butane working capacity isimproved by adding a heat-storage material.

A heat-storage type adsorbent composition was prepared in the samemanner as in Example 1, except using eicosane instead of then-octadecane used as a phase-change material in Example 1. Using theresulting heat-storage type adsorbent composition, the butane workingcapacity was measured in the same manner as in Example 2. The resultshowed an improved butane working capacity over that of ComparativeExample 1.

A heat-storage type adsorbent composition was prepared in the samemanner as in Example 1, except using caprylic acid instead of then-octadecane in Example 1. Using the resulting heat-storage typeadsorbent composition, the butane working capacity was measured in thesame manner as in Example 2. The result showed an improved butaneworking capacity over that of Comparative Example 1.

A heat-storage type adsorbent composition was prepared in the samemanner as in Example 1, except using methyl palmitate instead of then-octadecane in Example 1. Using the resulting heat-storage typeadsorbent composition, the butane working capacity was measured in thesame manner as in Example 2. The result showed an improved butaneworking capacity over that of Comparative Example 1.

EXAMPLE 3

To 5 g of melamine power, were added 6.5 g of 37% aqueous formaldehydesolution and 10 g of water, the pH of the mixture was adjusted to pH 8,and the temperature was raised to about 70° C., giving an aqueoussolution of melamine/formaldehyde initial condensate. Separately, asolution of 70 g of n-octadecane as a phase-change compound and 1.4 g ofcastor oil as an anti-supercooling agent was added to 100 g of asolution of a styrene anhydride copolymer sodium salt adjusted to pH 4.5while intensely stirring, and emulsification was conducted until theparticle diameter became about 10 μm. Encapsulation was conducted byadding the total amount of the above aqueous solution ofmelamine/formaldehyde initial condensate to the thus obtained emulsifiedsolution, stirring at 70° C. for two hours, and adjusting the pH to pH9.

After completion of the reaction, the capsules were filtered out bysuction and dried, giving capsules having a particle diameter of about15 μm. 25 parts by weight of these capsules and 5 parts by weight ofbinder (carboxymethylcellulose) were dispersed in a small amount ofwater, 100 parts by weight of pulverized activated carbon having aparticle diameter of 1 mm to 3 mm was added to the obtained dispersion.The resulting mixture was uniformly mixed, and then dried at 90° C.,obtaining the objective heat-storage type adsorbent composition whereinthe microcapsules were adhered to the surface of the activated carbon.

The activated carbon used was 1 mm to 3 mm pulverized coal activatedcarbon having a specific surface area of 1500 m²/g, a pore capacity of0.96 ml/g, and an average pore diameter of of 13 Å.

EXAMPLE 4

The butane working capacity of the heat-storage type adsorbentcomposition obtained in Example 3 was measured by the following method.The above heat-storage type adsorbent composition was placed in aone-liter metal canister, 99% n-butane was adsorbed by downflowing it tothe canister at one l/min at 25° C., and supply of the butane wasstopped when the concentration of the butane at the outlet reached 5000ppm. Air was then let into the canister at room temperature at 15 l/minfor 20 minutes by upflowing it to the canister to desorb the n-butane.Such absorption and desorption operations were repeated. The butaneworking capacity was determined based on the mean values of the amountsof the 4 _(th), 5 _(th) and 6 _(th) absorption and desorption cycles.

As a result, the butane working capacity was found to be 62.5 g/l for aone-liter canister case. The maximum temperature while adsorbing was 57°C. in the center of the case, the minimum temperature while desorbingwas 18° C. in the center of the case.

The heel amount (the amount of butane left in pores after desorption)after the 6 _(th) desorption was 30.8 g/l.

COMPARATIVE EXAMPLE 2

The butane working capacity of the activated carbon used in Example 3only was measured in the same manner as in Example 4, and found to be56.3 g/l for a one-liter canister case. The maximum temperature whileadsorbing was 73° C. in the center of the case, the minimum temperaturewhile desorbing was 14° C. in the center of the case.

The heel amount (amount of butane left in pores after desorption) afterthe 6 _(th) desorption was 48.2 g/l.

As is clear from the above results, the adsorbent composition comprisesheat-storage material containing a phase-change material, a canistercomprising the composition can improve the butane working capacity,reduce the temperature rise ratio during absorption, and enhance thedesorption ability (reduction of heel amount), and thereby the emissionamount of vapor gas can be reduced.

EFFECT OF THE INVENTION

When a canister containing the latent-heat storage type adsorbentcomposition of the present invention is used, the adsorption heatgenerated when fuel vapor is adsorbed by an adsorbent is transferred toa heat-storage material containing a phase-change material, and storedas latent heat in the heat-storage material. Therefore, the ratio oftemperature rise in the adsorbent is lessened and the capability foradsorbing fuel vapor is significantly improved.

When the fuel vapor is desorbed from an adsorbent, the heat stored inthe heat-storage material is transferred to the adsorbent, and thereforereduction of the temperature of the adsorbent is lessened. This furtherimproves the fuel vapor desorption ability.

Therefore, the latent-heat storage type adsorbent composition forcanisters of the invention exhibits a significantly improved ability foradsorbing and desorbing fuel vapor compared to conventional adsorbentsfor canisters and adsorbents for canisters containing a high specificheat.

Furthermore, because the temperature rise of a canister caused by theheat generated during adsorption is reduced, inexpensive materialshaving a lower heat resistance can be used for a canister case. Thismakes it possible to provide a miniaturized canister at low cost.

1: A latent-heat storage type adsorbent composition for canisterscomprising an adsorbent and a heat-storage material; the adsorbent beingcapable of adsorbing fuel vapor, the heat-storage material comprising amicroencapsulated phase-change material, the phase-change materialabsorbing or releasing latent heat in response to temperature change. 2:A latent-heat storage type adsorbent composition for canisters accordingto claim 1, wherein the adsorbent is activated carbon, activated aluminaor a mixture thereof. 3: A latent-heat storage type adsorbentcomposition for canisters according to claim 1, wherein the averageparticle diameter of the heat-storage material is about 1/1000 to about1/10 of that of the adsorbent. 4: A latent-heat storage type adsorbentcomposition for canisters according to claim 3, wherein the averageparticle diameter of the adsorbent is about 1 μm to about 10 mm. 5: Alatent-heat storage type adsorbent composition for canisters accordingto claim 1, wherein the average particle diameter of the heat-storagematerial is about 0.1 to about 500 μm. 6: A latent-heat storage typeadsorbent composition for canisters according to claim 1, wherein theheat-storage material is adhered to and/or deposited on the surface ofthe adsorbent. 7: A latent-heat storage type adsorbent composition forcanisters which is in a form of a molded article comprising alatent-heat storage type adsorbent composition for canisters accordingto claim 1 and a binder. 8: A latent-heat storage type adsorbentcomposition for canisters according to claim 7, wherein the moldedarticle is in at least one shape selected from the group consisting ofpellet, disc and block. 9: A method for producing a latent-heat storagetype adsorbent composition for canisters according to claim 1 whereinthe heat-storage material is adhered to and/or deposited on the surfaceof the adsorbent. 10: A method for producing a latent-heat storage typeadsorbent composition for canisters according to claim 1 wherein theheat-storage material is electrostatically adhered to and/or depositedon the surface of the adsorbent. 11: A method for producing alatent-heat storage type adsorbent composition for canisters accordingto claim 1 wherein the heat-storage material and the adsorbent areuniformly mixed. 12: A method for producing a latent-heat storage typeadsorbent composition for canisters according to claim 1 wherein aslurry obtained by suspending the heat-storage material in a liquidmedium is mixed with the adsorbent, and the mixture is then dried. 13: Amethod for producing a latent-heat storage type adsorbent compositionfor canisters comprising: suspending a heat-storage material containinga microencapsulated phase-change material in a liquid medium to give aslurry, the phase-change material capable of absorbing or releasinglatent heat in response to temperature change, and spraying a liquidmixture containing the slurry and, if necessary, a binder, on thesurface of the fuel vapor adsorbent. 14: A method for producing alatent-heat storage type adsorbent composition for canisters comprising:molding a heat-storage material containing a microencapsulatedphase-change material capable of absorbing or releasing latent heat inresponse to temperature change to produce a molded article, anduniformly mixing a fuel vapor adsorbent and the molded article. 15: Amethod for producing a latent-heat storage type adsorbent compositionfor canisters comprising: uniformly mixing a fuel vapor adsorbent, apowdery heat storage material containing a microencapsulatedphase-change material capable of absorbing or releasing latent heat inresponse to temperature change or a slurry suspending the powdery heatstorage material in the liquid medium, a binder and water, and moldingthe resultant mixture to form a desired shape. 16: A latent-heat storagetype adsorbent composition for canisters obtained by the methodaccording to claim
 13. 17: A canister for preventing fuel vaporizationin which the latent-heat storage type adsorbent composition of claim 1is placed in a canister case. 18: A latent-heat storage type adsorbentcomposition for canisters obtained by the method according to claim 14.19: A latent-heat storage type adsorbent composition for canistersobtained by the method according to claim 15.